1. Field of the Invention
The present invention relates to a technology for manufacturing, for instance, a tuning fork type piezoelectric resonator using a piezoelectric substrate made of, for instance, quartz crystal or the like.
2. Description of the Related Art
The tuning fork type quartz resonator has long been adopted as a signal source for pacing of a wrist watch owing to its compactness, inexpensiveness and low power consumption, and the uses thereof are still expanding. The CI (crystal impedance) value of this quartz resonator is required to be as small as possible for the purpose of reducing the power loss, and therefore a quartz resonator of which oscillation efficiency is enhanced by forming a groove therein has been used for this purpose.
As shown in FIG. 9, the quartz resonator is provided with a pair of vibrating arms 2a and 2b in a base 1, and the grooves 31 and 32 are respectively arranged on both main surfaces of the respective vibrating arms 2a and 2b. Excitation electrodes (not shown) for exciting tuning fork vibration based on bending vibration are formed in these grooves 31, 32 and the respective vibrating arms 2a, 2b. 
The above-described quartz resonator is manufactured according to the following process (refer to Patent Document 1). FIGS. 10A to 10E and FIGS. 11F to 11I are views showing the manufacturing processes of the cross sectional portion along A-A line in FIG. 9. First, after polishing and cleaning a cutout quartz wafer 10, a metal film 11 is formed by a sputtering method (FIG. 10A). A film in which gold (Au) is stacked on a backing film of, for instance, chromium (Cr) is used as the metal film 11.
Then, after applying a photoresist on such a metal film 11 by a spray method for instance (FIG. 10B), the photoresist is exposed and developed in a manner to obtain a pattern having the shape of the quartz piece, namely, a tuning fork shape pattern, so that a tuning fork shaped resist film 12 is formed (FIG. 10C). Thereafter, the portion of the metal film 11 not covered with the resist film 12 is removed by etching using, for instance, a potassium iodide (KI) solution, and the whole resist film 12 remaining on the quartz wafer 10 is removed (FIG. 10D).
Next, the photoresist is applied to the whole surface of the quartz wafer 10 by, for instance, a spray method, and a resist film 13 is formed (FIG. 10E). Then, a resist pattern is formed by lithography so that the outside shape of the quartz piece is preserved, and the portions corresponding to the grooves 31 and 32 shown in FIG. 9 are opened (FIG. 11F).
Thereafter, while the resist film 13 is kept on the metal film 11, wet etching is conducted by immersing the quartz wafer 10 into hydrofluoric acid which is an etching solution using the metal film 11 as a mask to form the outside shape of the quartz piece (FIG. 11G). 14 and 15 in FIGS. 11F to 11I are portions corresponding to the vibrating arms 2a and 2b, and 16, 17 in FIGS. 11F to 11I are outer frame portions described for convenience sake of taking out a sheet of quartz piece from the quartz wafer. Then, the metal film 11 at the portions corresponding to the grooves 31 and 32 shown in FIG. 9 is removed by etching in a potassium iodide (KI) solution using the resist film 13 as a mask (FIG. 11H). Then, wet etching is performed by immersing the quartz wafer 10 into hydrofluoric acid. By these processes, the grooves 31 and 32 are formed on both main surfaces of the quartz piece 10 (FIG. 11I).
Then, after removing all of the resist film 13 and the metal film 11 remaining on the surface of the quartz wafer 10, an electrode pattern is formed on the surface of the quartz wafer 10. The process of forming the electrode pattern will be explained as follows. First a metal film 17 to be an electrode is formed on both surfaces of the quartz wafer 10 by sputtering method (FIG. 12J). A film in which gold (Au) is stacked on a backing film of, for instance, chromium (Cr) is used as the metal film 11.
Next, the photoresist is applied to such a metal film 17 by the spray method (FIG. 12K). A resist film 18 excepting the resist film 18 that will be an electrode pattern is removed by a photolithography method (FIG. 12L). Thereafter, an electrode pattern is formed by etching the metal film 17 at the portion where the resist film 18 has been removed (FIG. 12M). Then, all of the resist film 18 remaining on the quartz wafer 10 is removed (FIG. 12N). Thus, excitation electrodes are formed in the grooves 31 and 32, and the respective vibrating arms 2a and 2B.
At the tip portions of the excitation arms 2a and 2b (areas shown by 33 in FIG. 9), the surfaces of the metal film (excitation electrode) 17 formed on the tips are shaved by laser or the like so as to adjust oscillation frequency by adjusting the thickness thereof.
There are however the following disadvantages in the process of adjusting the oscillation frequency. In a process shown in FIG. 12J, the film thickness of the metal film 17 formed on the surface of the quartz wafer 10 is very thin, being, for instance, 0.1 □m in thickness. Therefore, when the metal film 17 is used as it is for an adjustment film to adjust the oscillation frequency, it has the disadvantage of reduction in yield due to extreme narrowness of the allowable adjustment range in frequency.
As a countermeasure for this disadvantage, it is conceivable to make the thickness of the metal film 17 great in the process shown in FIG. 12J. However, it needs increased amount of metal to be used, and especially when gold (Au) is used as the metal, it has the disadvantage of increase in cost.
Furthermore, in Patent Document 2, it is described that following to the process of forming a metal film having the shape corresponding to the outside shape of the piezoelectric oscillating piece, a resist mask having an aperture only on the tip of the piezoelectric oscillating piece is formed, and a metal film made of gold (Au) for frequency adjustment is formed on the metal film by the field plating method in a manner to expose to the aperture. This requires a new specific process for forming a metal film for frequency adjustment, after forming the aperture on the resist mask, which result in an increase in the number of the total processes, and thus bringing a disadvantage of making the manufacturing process intricate.
In Patent Document 3, it is described that an electrode material and a photoresist are formed on the surface of a quartz plate in this order, and after etching the electrode material using an exposed resist film as a mask, the quartz plate is etched using this electrode material as a mask, and this electrode material is used as an excitation electrode, leaving the electrode material on the surface of the quartz plate. However, there is no description of the film for frequency adjustment.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2002-76806 (columns 0094 to 0113, FIGS. 9 to 13)
[Patent Document 2]
Japanese Patent Application Laid-open No. 2006-108824 (columns 0036 to 0037, FIG. 4)
[Patent Document 3]
Japanese Patent Application Laid-open No. Sho. 61-13704 (FIG. 2)